Atmospheric conditions needed for fat snow flakes



Where I live is getting some crazy snowfall right now. There isn’t that much snow but the flakes are larger than golf balls and very fluffy, not like hail. What are the specific atmospheric conditions that make the flakes so much larger than usual? (See my previous posts for a video example).

In: Physics

Ice crystals get stuck to dust particles and then stick together to form a snowflake. When exactly the snowflake falls depends on multiple factors like size, temperature, and turbulence.

For snowflakes to get that big before dropping, the temperature would need to be barely freezing (probably *slightly* above freezing), low wind conditions, high amount of moisture in the air, and a high variance in atmospheric (barometric) pressure.

The near-freezing temperature combined with the high variance in barometric pressure means the temperature can cycle between above and below freezing. This melts the edge of a forming snowflake which makes it more ‘sticky’ and makes these snowflakes stick together. The high moisture allows for these larger flakes to form more quickly, and the low wind keeps the snowflakes from “shaking” out of the cloud before they reach their critical mass.

Joel Gratz, founder of OpenSnow, does a good job of explaining it here